1. Field of the Invention
The present invention relates to a wireless access point apparatus which carries out wireless communications with a plurality of wireless communicating apparatuses located in a service area thereof, a wireless LAN system incorporating the wireless access point apparatus, a wireless communicating method, a program for implementing the method, and a storage medium storing the program.
2. Description of the Related Art
A wireless LAN system in general is comprised of wireless mobile terminals which carry out data communications with each other via a wireless transmission channel or a wired LAN, and an access point which carries out wireless communications with a plurality of wireless mobile terminals in a service area of the system and executes a bridging function of connecting a wireless area and a wired LAN to each other.
IEEE 802.11 as a wireless LAN standard defines a wireless access protocol between a wireless access station and a terminal station. In this standard, a DCF (Distributed Coordination Function) as a collision-type random access function based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) and a PCF (Point Coordination Function) as an optional function are defined. The PCF is a function of periodically setting a CFP (Contention Free Period) on a wireless channel used for communications with the terminal station by the wireless access station and carrying out non-collision type access during the CFP by polling (refer to Japanese Laid-Open Patent Publication (Kokai) No. 2000-253017, for example).
Referring next to FIG. 18, a description will be given of a wireless transmission frame in a wireless LAN system using the conventional PCF and DCF. FIG. 18A is a diagram showing an example of the structure of a wireless transmission frame in a wireless LAN system using the conventional PCF and DCF, and FIG. 18B is a diagram showing an example of the structure of a wireless transmission frame in the case where only the DCF is used.
In the wireless transmission frame in the wireless LAN system using the conventional PCF and DCF, as shown in FIG. 18A, a transmission frame which comes every predetermined transmission frame period 205 is defined, and a beacon information transmission area 201 and a data transmission area 204 are provided in the transmission frame. Here, the frame structure is shown in a convenient form, but this structure form should not necessarily be used.
The beacon information transmission area 201 is used to notify a frame period and network common information, and is located at the top of the frame. The beacon information is a management area (frame synchronization area) including frame synchronization information, and is assigned to each communicating station constituting a network. For the beacon information, an arrangement has been devised which prevents collision of data sent from a plurality of communicating stations.
The beacon information includes information about an area for which bandwidth reservation has been made in the data transmission area 204, and an asynchronous transmission area. Specifically, the data transmission area 204 is comprised of a bandwidth-reserved transmission area (PCF) 202 set as the need arises, and an asynchronous transmission area (DCF) 203 outside the bandwidth-reserved transmission area (PCF) 202. The bandwidth-reserved transmission area (PCF) 202 is also referred to as a non-collision area, which can be exclusively used by an arbitrary wireless terminal by polling from a wireless access point apparatus. The asynchronous transmission area (DCF) 203 is also referred to as a collision area, which is a time area during which communication is carried out at random according to a predetermined procedure.
The bandwidth-reserved transmission area (PCF) 202 is provided as an optional function; in the case where the amount of data in a wireless terminal is assumed to be small, or in the case where communication is irregular, the transmission efficiency is deteriorated by polling a plurality of wireless terminals, and hence in an ordinary wireless access point apparatus, a normal wireless transmission frame comprised of the beacon information transmission area 201 and the asynchronous transmission area (DCF) 203 as shown in FIG. 18B is used.
As mentioned above, the asynchronous transmission area (DCF) 203 is used for transmission of data to and from the wireless access point apparatus and a plurality of wireless terminals connected to or accommodated in the wireless access point apparatus. However, as to wireless terminals connected to or accommodated in the service area, the larger the number of wireless terminals connected to one access point, the higher the probability of collision, and hence the number of times a wireless medium message channel medium is assigned to one wireless terminal per unit time is decreased to deteriorate the throughput in sending and receiving data. When activated, the wireless access point apparatus sends a beacon according to a wireless transmission frame structure fixed in advance and dedicated to the apparatus.
However, when activated, the conventional wireless access point apparatus sends a beacon according to a wireless transmission frame structure fixed in advance and dedicated to the apparatus, and hence in the case where a wireless terminal which desires regular transmissions and has a wireless function of sending and receiving data in real-time, such as a digital video apparatus, is connected to or accommodated in the wireless access point apparatus which permits only the asynchronous transmission area (DCF), the above wireless terminal has to carry out random access communications with the other wireless terminals according to a predetermined procedure while conflicting with them during the asynchronous transmission area (DCF), and as a result, data cannot be surely transmitted in real-time.
Also, in the case where not a wireless terminal which desires regular transmissions and has a wireless function of sending and receiving data in real-time, such as a digital video apparatus, but wireless terminals which do not place emphasis on real-time transmissions are connected to or accommodated in an access point apparatus which permits both the bandwidth-reserved transmission area (PCF) and the asynchronous transmission area (DCF), the bandwidth-reserved transmission area (PCF) which is not used is incorporated into the wireless transmission frame structure, and hence the period of time which is assigned to the asynchronous transmission area (DCF) is decreased. As a result, when access from wireless terminals occurs concentratedly, the throughput is deteriorated.